1. Field of the Invention
This invention relates to a method for producing an optical power sampling device. Accordingly, it is a general object of this invention to provide new and improved methods of such character.
2. Description of the Prior Art
In optical communication systems, it is desirable to stabilize the output of an optical source against variations due to degradation during life or due to temperature changes. Stabilization is particularly needed for laser diodes, but may also be utilized for light emitting diodes.
To stabilize the output of these devices, a sample of the power carried along the transmission medium is needed to provide a signal input to a feedback control loop, which then adjusts the diode electrical bias current, and possibly the drive signal, to maintain either a constant average optical power or a constant peak optical signal amplitude.
A prior art method for sampling the optical output power of a diode laser makes use of the power radiating from the rear face of a laser chip. A disadvantage is that the power emitted from the rear face is not necessarily linearly proportional to the power coupled into the system waveguide (optical fiber). Even when each of two identical optical fibers are coupled to the front and rear faces of the laser, the optical modes coupled into each of the fibers are a function of their orientation with respect to the front and rear laser radiation patterns. It is extremely difficult in practice to achieve precisely the same alignment.
"Effects of beam displacement and front and back mistracking of junction lasers on lightwave transmitter output stability", by F. S. Chen, M. A. Karr, and P. W. Shumate, July 15, 1978/Vol. 17, No. 14/Applied Optics, pp. 2219-2223, discusses the tapping of an optical fiber for feedback control of a laser. In this referenced paper, the optical fiber tap, functioning as a beam splitter, extracts a portion of the guided optical power in the fiber. It consists of two fibers having angle-polished ends placed close to each other but separated by a small air gap. The glass-to-air interfaces reflect about 10% of the guided power away from the fiber axis, and this reflected light is intercepted by a P-I-N photodiode.